Insulating lid for aluminum production cells

ABSTRACT

An aluminum production cell includes an inert anode and an insulating lid comprising alumina and at least one metal fluoride. The insulating lid preferably comprises about 35-90 wt. % of a mixture of sodium fluoride and aluminum fluoride and about 10-65 wt. % alumina.

PENDING RELATED APPLICATION

[0001] This application is a continuation-in-part of Slaugenhaupt et al.U.S. Ser. No. 09/732,716, filed Dec. 8, 2000.

FIELD OF THE INVENTION

[0002] The present invention relates to aluminum production cells and,more particularly, relates to insulating lids for aluminum productioncells having inert anodes.

BACKGROUND INFORMATION

[0003] The energy requirements and cost efficiencies of aluminumsmelting cells can be significantly reduced with the use of inert,non-consumable and dimensionally stable anodes. Replacement oftraditional consumable carbon anodes with inert anodes allows a highlyproductive cell design to be utilized, thereby reducing capital costs.Significant environmental benefits are also possible because inertanodes produce essentially no CO₂ or CF₄ emissions.

[0004] The successful retrofit of conventional consumable anode Hallaluminum production cells with inert anodes requires extremely stablecell operation with respect to bath chemistry and heat balance. Toachieve cell stability, several operating parameters must be controlled.The reduced capacity of inert anode cells to produce heat in comparisonwith conventional cells having consumable carbon anodes requires heatlosses to be minimized in order to maintain an adequate heat balance andstable operation. The amount of alumina dissolved in the bath must becontrolled within a very narrow range, e.g., about 7 to 8 percent, whichis essential for low corrosion rates of the inert anodes and to preventdeposits of excess alumina on the cathode surface. At an operatingvoltage equivalent to a conventional carbon anode cell, an inert anodecell will generate less heat due to a high decomposition potential.Control of the cell temperature is crucial because the aluminasaturation level of the bath is proportional to the bath temperature.Temperature control is also crucial to the formation and stability ofthe protection layer of frozen bath or ledge along the sidewalls of thecell. In a conventional Hall cell, this ledge prevents horizontalelectrical currents between the anode and cathode as well as protectingthe sidewall lining material from erosion. In a cell with inert anodes,this ledge may also protect the inert anodes from reduction bycarbonaceous material of the cell lining.

[0005] A need exists for increased insulation in inert anode aluminumproduction cells in order to reduce heat losses from the cells. Thepresent invention has been developed in view of the foregoing.

SUMMARY OF THE INVENTION

[0006] The present invention improves the operating stability of aninert anode aluminum production cell by controlling the heat lossthrough a top insulating cover. The top cover is the primary area thatheat loss can be regulated during normal operation. Reduction in heatloss is preferably obtained by utilizing a multiple layer insulationsystem on top of the inert anodes. The initial layer of insulationpreferably comprises fabricated blocks of a cryolite and aluminamixture. These blocks, which are resistant to attack from the bathfumes, may shield and support subsequent layers of high temperatureinsulation. To further protect the upper layers of insulation, asemi-gas-tight barrier may be created by filling voids between theblocks with loose refractory material, such as coarse tabular aluminachoked by a smaller particle size alumina material. The heat balance ofthe cell may be maintained, e.g., by adding or removing layers ofinsulation from the top of the cell.

[0007] An additional application for the preformed insulating blocks ofthe present invention is during startup of an inert anode cell. Theblocks may be used as an inner sidewall between a carbon cell lining andthe anodes. This inert lining provides horizontal electrical insulationand protect the anodes from carbothermic reduction until a permanentcryolite ledge is formed.

[0008] An aspect of the present invention is to provide an aluminumproduction cell insulating lid assembly comprising at least onepreformed refractory block comprising Al₂O₃ and at least one materialselected from NaF, AlF₃, CaF₂ and MgF₂.

[0009] Another aspect of the present invention is to provide an aluminumproduction cell including a cathode, at least one anode, and aninsulating lid above the cathode and anode(s). The insulating lidcomprises at least one preformed refractory block including Al₂O₃ and atleast one material selected from NaF, AlF₃, CaF₂ and MgF₂.

[0010] These and other aspects of the present invention will be moreapparent from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a partially schematic sectional side view of an inertanode aluminum production cell including an insulating lid in accordancewith an embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0012]FIG. 1 illustrates an insulating lid 10 of an aluminum productioncell 11 in accordance with an embodiment of the present invention. Theinsulating lid 10 includes preformed refractory blocks 12 which arepositioned above inert anodes 14. The inert anodes 14 may be made of aceramic material as described in Dimilia et al. U.S. Ser. No.10/291,874, filed Nov. 8, 2002 or in Weirauch et al. U.S. Ser. No.10/291,874 filed Nov. 9, 2002; or a cermet material as described in Rayet al. U.S. Pat. No. 6,423,204 issued Jul. 23, 2002. The disclosures ofthe aforesaid patent applications and patent are incorporated herein byreference.

[0013] The preformed refractory blocks 12 preferably comprise from about35 to about 90 weight percent of at least one metal fluoride selectedfrom sodium fluoride, aluminum fluoride, and mixtures thereof and fromabout 10 to about 65 weight percent Al₂O₃. Compositions comprisingmixtures of sodium fluoride and aluminum fluoride together with aluminaare preferred. More preferably, the preformed refractory block comprisesabout 50 to 75 weight percent of a NaF/AlF₃ mixture, and the Al₂O₃comprises from about 25 to about 50 weight percent. The weight ratio ofNaF:AlF₃ preferably ranges from about 1:1 to about 2:1. A particularlypreferred weight ratio of NaF:AlF₃ is about 1.5:1. The NaF and AlF₃ maybe provided separately or together, e.g., in the form of syntheticcryolite. In addition to, or in place of, the NaF and AlF₃, othermaterials, such as calcium aluminate, CaF₂, and MgF₂, may be used. Forexample, the refractory block material may comprise crushed bath heldtogether with a suitable amount of binder, such as sodium aluminumtetrafluoride or alumina cement.

[0014] The preformed insulating blocks 12 may be formed by mixing, e.g.,the Al₂O₃, NaF/AlF₃ mixture and binder components in the desired weightratios, followed by pressing and heating to a temperature sufficient tobind the block together, e.g., 1,000° C. The thickness and shape of thepreformed insulating blocks 12 may be selected in order to providesufficient structural integrity and heat insulating properties. Forexample, thicknesses of from less than 1 inch to greater than 1 foot maybe used.

[0015] After the preformed refractory blocks 12 are positioned above theinert anodes 14, spaces around the preformed refractory blocks 12 may bepacked with loose insulation 16, such as particles, fibers, and thelike. For example, the loose insulation may comprise tabular alumina,crushed bath and/or alumina particles. As a particular example, theloose insulation may comprise from 75 to 90 weight percent ESP dust andfrom 10 to 25 weight percent crushed bath.

[0016] Additional layers of insulation 18 may be positioned above thepreformed refractory blocks 12, as shown in FIG. 1. The additionalinsulation 18 may include alumina particles, ceramic fiber, fiberfrax,fiberglass, or any other suitable material.

[0017] In addition to their use as insulating lids, the preformedrefractory blocks of the present invention may be used as sidewallliners of the cell. FIG. 1 illustrates a preformed sidewall block 20positioned inside the sidewall 22 of the cell. The sidewall block 20 maybe the same composition as the insulating blocks 12. In this case, thepreformed sidewall insulation 20 may act as a sacrificial material whichmay be at least partially consumed during operation of the cell.

[0018] The aluminum production cell 11 also includes a chamber 30containing a molten salt bath 32 comprising cryolite and dissolvedalumina; and a cathode 34 spaced from the inert anodes 14. When anelectric current passes between the inert anodes 14 and the cathode 34,aluminum 36 is produced at the cathode 34 and oxygen is produced at theanodes 14.

[0019] Whereas particular embodiments of this invention have beendescribed above for purposes of illustration, it will be evident tothose skilled in the art that numerous variations of the details of thepresent invention may be made without departing from the invention asdefined in the appended claims.

What is claimed is:
 1. An aluminum production cell comprising at leastone inert anode, a chamber containing a molten salt bath contacting saidinert anode, a cathode spaced from said inert anode, and an insulatinglid covering said chamber said insulating lid comprising a preformedrefractory block containing about 10-65 wt. % alumina and about 35-90wt. % of at least one metal fluoride selected from sodium fluoride,aluminum fluoride, and mixtures thereof.
 2. The aluminum production cellof claim 1, wherein alumina comprises about 25-50 wt. % of the preformedrefractory block.
 3. The aluminum production cell of claim 1, whereinsaid insulating block comprises sodium fluoride and aluminum fluoride.4. The aluminum production cell of claim 1, wherein said insulatingblock comprises sodium fluoride and aluminum fluoride in an NaF:AlF₃weight ratio of about 1:1 to about 2:1.
 5. The aluminum production cellof claim 4, wherein the NaF:AlF₃ weight ratio is about 1.5:1.
 6. Thealuminum production cell of claim 1, wherein said refractory blockfurther comprises at least one metal fluoride selected from calciumfluoride and magnesium fluoride.
 7. The aluminum production cell ofclaim 1, wherein said inert anode comprises a ceramic or cermetmaterial.
 8. The aluminum production cell of claim 1, wherein said inertanode comprises nickel oxide and iron oxide.
 9. The aluminum productioncell of claim 1, further comprising loose insulation around a portion ofthe preformed insulating block.
 10. The aluminum production cell ofclaim 9, further comprising at least one additional layer of insulatingmaterial above the preformed insulating block.
 11. The aluminumproduction cell of claim 1, wherein the preformed insulating block ispositioned above at least one anode of said cell.
 12. A process forproducing aluminum in a cell comprising at least one inert anode, achamber containing a molten salt bath containing alumina, a cathodespaced from said inert anode, and an insulating lid covering saidchamber, said process comprising (a) passing an electric current betweensaid anode and said cathode, thereby to produce aluminum at the cathode,and (b) conserving heat in said chamber by providing said insulating lidwith a composition comprising about 10-65 wt. % alumina and about 35-90wt. % of at least one metal fluoride.
 13. The process of claim 12,wherein said insulating lid comprises at least one metal fluorideselected from sodium fluoride, aluminum fluoride, magnesium sluoride,calcium fluoride, and mixtures thereof.
 14. The process of claim 12,wherein said insulating lid comprises a preformed refractory blockcomprising about 50-75 wt. % of a mixture of sodium fluoride andaluminum fluoride and about 25-50 wt. % alumina.
 15. The process ofclaim 14, wherein said mixture comprises NaF and AlF₃ in an NaF:AlF₃weight ratio of about 1:1 to about 2:1.